A username is simply your name or nickname used to identify you on a computer or network. No two people or accounts can have the same username, just like an email account.
A username could be simply your first name or nickname on your computer - but would probably need to be something more than that on a network, due the chances that there are many people with the same first name or even nickname. The combination of your username along with your password identifies YOU, so that you have access to YOUR data (email, files, etc.) and at the same time, prevents other people from accessing it.
Most times, you get to select your own username and password. Some networks assign a username - often like "firstname.lastname", company ID number, and email accounts like "[email protected]
" You may even prohibited from picking an easy password - by needing to meet certain criteria such as using number, capital letters, special symbols, minimum length, etc. In any case, pick whatever you like and let the computer or system tell you if it is sufficient or not.
I hope this helps - please rate my answer "4 thumbs up" if it was. Good luck!